1. Field of the Invention
The present invention relates to liquid crystal displays, and more particularly, to a color liquid crystal display utilizing selective reflection of cholesteric liquid crystal.
2. Description of the Background Art
A conventional field effect mode liquid crystal display has a merit that a thin display with low power consumption can be formed. A typical display sandwiches a chiralnematic liquid crystal layer with orthogonal polarizers, as described in Japanese Patent Publication No. 51-13666. FIG. 1 shows the device disclosed in this publication, and includes a polarize plate 101, an analyze plate 105, and an electro-optic cell 106 therebetween. Electro-optic cell 106 includes a pair of electrodes 102 and 104, and a nematic liquid crystal layer 103 therebetween. This device transmits or blocks light incident upon polarize plate 101 according to the intensity of the electric field applied between electrodes 102 and 104. This device has a narrow angle of view, and it is difficult to achieve contrast available for practical usage without a transmission type display. These disadvantages result mainly from loss of light by the polarize plate and angle of view dependency upon the axis of polarization. In general, there is loss of light exceeding 50% even if the twist angle of a nematic layer is increased to improve characteristics such as response and angle of view dependency in a display employing birefringence. If color is to be displayed with such a device, light loss is further aggravated due to a filter which will be used. Also, a transmission type display requires illuminating means at the rear thereof, causing increase in the thickness of size and power consumption of the display. This will degrade the merits of a liquid crystal display.
A liquid crystal display in which dye is added to a chiralnematic liquid crystal, i.e. the so-called guest-host type or white tailor type liquid crystal display, is known. Such a display requires dye that is bright in color and that has affinity for liquid crystal molecules because electro-optical characteristics inherent in liquid crystal is limited by the dye. However, a combination of dye and liquid crystal with superior electrical characteristics has not yet come into practical use. A short pitch cholesteric liquid crystal display taking advantage of coloring according to the temperature dependency of the liquid crystal is proposed, but lacks stable color display and high response rate of display.
Another system is known that uses light scattering by focal conic texture or Williams domain texture. This system also allows reflective type display. However, this system takes advantage of electro-optical effect depending upon natural chirality of liquid crystal, so that the ability to return to the former state after electric field removal depends upon the rearranging ability of the liquid crystal itself. Therefore, this system has low response speed. An improved optical material used in this system is disclosed in Japanese Patent National Publication No. 61-502128 and Japanese Patent Laying-Open No. 62-2231. These publications describe the technique to confine liquid crystal within a three dimensional network structure of porous polymeric resin to form a capsule. However, usage of such an encapsuled material results in low contrast since the change of the amount of light transmission controlled by liquid crystal is limited by the resin component. There was also a disadvantage that a bright color display where a non-lighted area exhibits achromatic color and a lighted area exhibits high saturated color could not be achieved because the color of a color filter will always be slightly observed due to the transmitted light and resin.
As described above, conventional liquid crystal displays had low usage efficiency of light and many problems in material or the like to be solved.